1. Field of the Invention
The present invention relates to a control device for a resistance welder wherein, in a resistance spot welder in which an operation of pressurizing an electrode is conducted by an electric servo motor. An amount of thermal expansion of a welded part due to production of a weld nugget is detected by reading an encoder which is incorporated in the servo motor, thereby ensuring the welding quality or judging the welding quality.
2. Description of the Related Art
A technique of, in a resistance spot welding, conducting judgment on the quality of a welded part by detecting an amount of thermal expansion of the welded part during a current supply has been known. The amount of thermal expansion of a welded part is detected by detecting the amount of movement of a movable electrode during a welding. Conventionally, the amount of displacement of an electrode is detected in the following manner. An air pressure due to a minute stroke change of a rack and pinion mechanism or a movable electrode is detected as an output of a detector, or a position sensor such as a linear potentiometer is disposed between stationary and movable electrodes of a resistance welder.
Further, a system exists in which the pressurizing force is detected from the current supplied to a servo motor, and, when a weld nugget is produced by thermal expansion during a current supply, the position of an electrode is controlled by means of an encoder of the servo motor in accordance with the detected pressurizing force, thereby obtaining a desired welding quality.
The position sensor of the former technique is disposed in the vicinity of an electrode which conducts a welding operation, and hence has problems in that the position sensor is easily damaged during a welding operation by splashes scattering from a welded part, and that, depended on the shape of a workpiece, the position sensor may mechanically interfere with the workpiece.
In the latter technique or the electrode position control system using a servo motor, the position of an electrode can be made constant even when the amount of thermal expansion of a welded part is changed owing to production of a weld nugget of the base metal. However, the system cannot conduct processing in which, an amount of thermal expansion of a welded part is detected by reading an encoder which is incorporated in the servo motor, the welding quality is judged, monitoring results of the judgment are supplied to a host computer, the history is statistically processed to know the wearing state of an electrode chip, the tendency of success in welding position of a specific spot, etc.